1. Field of the Invention
The present invention relates generally to a wireless communication system. In particular, the present invention relates to an apparatus and method for processing digital signals in an Orthogonal Frequency Division Multiple Access (OFDMA) wireless communication system.
2. Description of the Related Art
A mobile communication system using cellular communication technology is the typical wireless communication system. Such a mobile communication system uses multiple access schemes in order to simultaneously communicate with a plurality of users. The multiple access schemes are classified as a Time Division Multiple Access (TDMA) scheme, a Code Division Multiple Access (CDMA) scheme, and a Frequency Division Multiple Access (FDMA) scheme. A communication system using the CDMA scheme, with the rapid progress of the CDMA technology, has developed from the early communication system mainly providing voice service, into an advanced communication system that is capable of transmitting high-speed packet data. However, in order to overcome the limited use of code sources in the CDMA scheme, an Orthogonal Frequency Division Multiple Access (OFDMA) scheme has been proposed.
An Orthogonal Frequency Division Multiplexing (OFDM) scheme is a scheme for transmitting data using multiple carriers, and is a kind of Multi-Carrier Modulation (MCM) scheme that parallel-converts a serial input symbol stream into parallel symbols and modulates the individual parallel symbols with a plurality of orthogonal subcarriers, that is, a plurality of orthogonal subcarrier channels, before transmission.
The MCM scheme was first applied to a military high-fidelity (HF) radio in the late 1950s. The OFDM scheme that overlaps a plurality of orthogonal subcarriers has developed since the 1970s, but has limitations in being applied to an actual system because of its difficulty in implementing orthogonal modulation between the multiple carriers. However, the rapid development of the OFDM scheme was triggered as Weinstein et al. announced in 1971 that OFDM-based modulation/demodulation could be efficiently processed using discrete Fourier transform (DFT). For reference, DFT transforms a time-domain signal into a frequency-domain signal, whereas inverse discrete Fourier transform (IDFT), an inverse process of DFT, transforms a frequency-domain signal into a time-domain signal. In addition, the possible use of a guard interval and the possible insertion of a cyclic prefix (CP) into the guard interval have reduced the defects of the system caused by multipath propagation and delay spread.
Owing to the development of the OFDM technology, the OFDM scheme is now widely being applied to such digital transmission technologies as digital audio broadcasting (DAB), digital television (DTV), wireless local area network (WLAN) and wireless asynchronous transfer mode (WATM). That is, the OFDM scheme, which was not widely used due to its hardware complexity, can now be implemented owing to the recent development of various digital signal processing technologies including fast Fourier transform (FFT) and inverse fast Fourier transform (IFFT). The OFDM scheme, being similar to the conventional frequency division multiplexing (FDM) scheme, is characterized by transmitting a plurality of subcarriers while maintaining orthogonality therebetween, thereby obtaining the optimal transmission efficiency during high-speed data transmission. In addition, the OFDM scheme has high frequency efficiency and is robust against multipath fading, so it can obtain the optimal transmission efficiency during high-speed data transmission. Moreover, because the OFDM scheme overlaps frequency spectrums, it has high frequency efficiency, is robust against frequency selective fading and multipath fading, can reduce inter-symbol interference (ISI) with the use of a guard interval, can enable simple hardware design of an equalizer, and is robust against impulse noises. Therefore, the OFDM scheme is now widely being applied to the communication system.
In the OFDMA communication system, a transmitter of a terminal performs IDFT on M-ary symbols, inserts a CP with an appropriate length into the IDFT-processed symbols, and delivers the CP-inserted symbols to its radio frequency (RF) stage. Generally, the transmitter uses IFFT to efficiently perform IDFT. A receiver removes a CP from a received signal and performs FFT on the CP-removed signal to offset the IFFT effect obtained during transmission.
In the commercial system, a fixed point algorithm is generally implemented instead of a floating point algorithm in order to reduce the production cost. Even in the IFFT or FFT process, the system generally performs IFFT or FFT using the fixed point algorithm. The fixed point algorithm expresses a signal with a particular number of data bits. In the process of performing quantization using the fixed point algorithm, a signal is expressed with a number of bits after its values following a particular digit below the decimal point are deleted, causing a quantization error.
Generally, when a quantization error is relatively lower than additive noise allowed in the system, the number of data bits is determined according to a signal-to-quantization noise ratio (SQNR) required by the system, because the full algorithm performance is less affected by the quantization error. That is, the actually implemented hardware allocates a number of data bits taking the SQNR required in the system into consideration, thereby implementing all data with the fixed point. For example, an OFDMA system using 1024 subcarriers uses 13 data bits for 40dB SQNR.
In the OFDMA system, subcarriers allocated to a terminal are subject to change, and a data bitwidth required for maintaining a constant SQNR due to the change in the allocated subcarriers is also subject to change. Accordingly, there is an increasing need for a system and method for varying the number of data bits according to the number of subcarriers allocated to the terminal during IFFT.